The present invention is directed to an improved method and apparatus for imaging objects with penetrating radiation.
The use of penetrating radiant energy is well known in the imaging of both inanimate and animate objects. As an example, systems of this type find important use in the inspection of objects to discover secreted contraband which may present a security threat.
One type of inspection system uses a flying spot of penetrating radiation to scan the object being inspected. After interaction with the object, the radiant energy may be incident on detector(s) which detect transmitted and/or scattered energy. For example, such a system is disclosed in U.S. Pat. No. Re 28,544, assigned to American Science and Engineering, Inc. This type of system is an improvement over the prior art, in that it allows operation without exposing personnel to excessive radiation. An additional advantage is that since the object may be divided into elemental frontal areas which ar scanned by the flying spot, and the detector signal may be time divided into intervals denoted as pixels which correspond to these elemental areas, the radiant energy which is incident on either a transmission or scatter detector at any given time may be referred to a particular elemental area of the object.
In such a flying spot inspection system, the apparatus for producing the flying spot includes a source of penetrating radiation, an absorber plate of highly absorbing material, preferably a high Z material such as lead which has a fixed slit therein, and a chopper wheel, which also must be made of highly absorbing material. The chopper wheel has radially directed slits, and when the chopper wheel is rotated, these slits rotate past the fixed slit in the absorber plate. As such rotation occurs, a flying spot of radiation is created along the direction of the fixed slit in the absorber plate, which is used to scan the object being inspected. Additionally, the system is arranged so that there is relative translation movement between the object and flying spot in a direction perpendicular to the scanning direction of the flying spot, so that the entire object is scanned in successive lines.
In the prior art flying spot scanning system, the flying spot is scanned in straight lines, which typically lie in the vertical or horizontal direction. This follows from the fact that the fixed slit in the absorber plate, which defines the scanning direction, is inevitably a straight line in the prior art. With such a system, in order to provide a scan line of adequate length, the radial slits in the chopper wheel, which must maintain coincidence with the projection of the radiation passing through the fixed slit as the wheel rotates, must be of substantial length.
While the flying spot scanning system described above has many advantages over the prior art, a problem with it is that because the radial slits must be of substantial length, it is necessary for the chopper wheel to have so much mass, that at the rotation speeds which are required for some applications, it may break apart. Additionally, the high mass of the wheel requires that a powerful motor be used, which may increase the expense of the equipment. By way of example, in a chopper wheel of current design, an annular disk or doughnut of lead which contains the radial slots is embedded in an aluminum wheel. For a typical application, the length of the radial slots must be 14.6 cm, which also defines the radius of the doughnut. Since the thickness of lead required is about 2.15 cm, the resultant weight of the lead doughnut is about 510 lbs (230 Kg.).